1. Field of the Invention
This invention relates to an image forming apparatus and method for changeable image forming modes between single color mode and a multiplicity of color modes, more particularly, this invention is concerned with a color image forming apparatus and method selectively operable in a multiple color image forming mode using two, three or four colors or in a single color image forming mode.
2. Discussion of the Background
In general, a number color image forming apparatuses, e.g. full color copy machines, full color printers, have selectable image forming modes which are a multiple color mode and a single color mode. This type of apparatuses can selectably output full color images or single color images like a monochrome images by only one machine.
For further user needs, the image forming apparatus are required to have both a high quantity of output images in full color images and a high output speed in single color image. Referring to FIG. 1 of the drawings, there is shown a specific construction of single color images forming apparatus of the type electrostatically forming a latent image on a photoconductive drum by controlling a laser in response to image signals.
As shown, a scanning section 200 includes a color CCD (Charge Coupled Device) image sensor 201 for reading an image of a document 100 and producing corresponding electric signals (i.e. image data). An image processing section 400 performs optimal image processing with the electric signal and converts the processed signals to color image signals representative of an image to record.
A writing section 300 receives the color image signals from the image processing section 400 to reproduce the document image on a photoconductive drum 5. Various conventional units for effecting electrophotography are arranged around the drum 5. Specifically, a main charger 510 uniformly charges the surface of the drum 5. The writing section 300 has a laser diode, which emits a laser beam. The laser beam is modulated by particular color image signals and scans the charged surface of the drum 5. As a result, a latent image representative of the document image is formed on the drum 5. The laser beam is incident to and deflected by a polygonal mirror 3 being rotated by a scanner motor 1. The laser beam from the mirror 3 is focused in the form of a spot on the drum 5 by conventional f-theta lenses 4.
A developing section 500 develops the latent image by a toner to produce a corresponding toner image. The developing section 500 has a yellow (Y) developing unit 500Y, a magenta (M) developing unit 500M, a cyan (C) developing unit 500C, and a black (Bk) developing unit 500Bk. For details of the developing section 500, reference may be made to, for example, U.S. Pat. No. 4,928,144, the disclosure of which is herein incorporated by reference. A cleaning section 520 removes the toner remaining on the drum 5 after the transfer of the toner image which will be described.
On the other hand, a sheet feed section 600 is loaded with a stack of sheets 601. A pick-up roller 602 feeds the sheets 601 one by one toward an image transfer section 700 via a transport roller 604 and a register roller 603. The image transfer section 700 is available in various forms as taught in, for example, U.S. Pat. Nos. 4,835,582, 4,862,214, 4,864,358 and 4,872,037 the disclosure of which is also incorporated by reference. While a transfer drum 701 drives the sheet 601, a transfer charger 702 transfers the toner image from the drum 5 to the sheet 601. To produce a black-and-white copy, a black toner image is formed on the drum 5 by the black developing unit 500Bk and then transferred to the sheet 601. Hence, the formation of a latent image, development of the latent image and transfer of the resulting toner image are each effected only once.
In the event of full color copying (i.e. printing), such a sequence of image forming steps are repeated with each of Y, M and C, i.e., three times in total or with each of Y, M, C and Bk, i.e., four times in total. The sheet 601 carrying the toner image thereon is separated from the transfer drum 701 at a sheet separating section 703 and then transported to a fixing section 800 to have the toner image fixed thereon. Finally, the sheet or copy 701 is driven out of the apparatus.
However, this type of a color image forming apparatuses carry out the above same sequence Y, M, C, Bk, four times in total in a full color image forming. Therefore, the image forming apparatus scarify the throughput to form full color images.
There is another known type of color image forming apparatuses. This type of color image forming apparatus employs an intermediate transfer belt or a sheet transfer belt where each developing unit Y, M, C, Bk are arranged in tandem.
Referring to FIG. 2 illustrates an example of this type of image forming apparatuses. An image transfer unit of this image forming apparatus has four photoconductive drums 1A, 2A, 3A and 4A on which color data of an image to be recorded with respect to yellow, magenta, cyan and black are optically written to form a latent image of a respective different color thereon and developing devices 1B, 2B, 3B and 4B each arranged in connection with each of the four photoconductive drums, respectively. The original color is separated to three colors, i.e., blue, green and red each of which is detected by a respective optical sensor. An image processor calculates color data of yellow, magenta, cyan and black on the basis of the luminous intensity of each of the separated three colors. The electrostatic latent images formed on each of the photoconductive drums 1A, 2A, 3A and 4A are developed by a toner of a corresponding color contained in the respective corresponding developing devices. A sheet is fed onto a transfer belt 5 from a paper container through a resist roller unit 7 and is conveyed by the transfer belt 5 which circulates along the photoconductive drums and comes in contact therewith one after another. The sheet is electrostatically charged and secured to the conveyor belt surface. The visible toner image formed on each of the photoconductive drums 1A, 2A, 3A and 4A are transferred to the sheet by a function of a corresponding transfer charger 1C, 2C, 3C or 4C disposed behind the transfer belt 5 at the position of each photoconductive drum in such a manner that the different color toner images are superposed one above the other to form an image of multi-colors on the record sheet. The full color image is subsequently fixed on the sheet by a fixing roller unit 8.
The color image forming apparatus illustrated in FIG. 2 has an image reading unit 9 disposed on the apparatus body. The color of an original (not shown) to be copied is optically separated to the above mentioned three colors and read by three CCDs 10, respectively, arranged in the image reading unit 9. An image processor device (not shown) calculates image color data for each color on the basis of the output signal from each of the three CCDs 10. A laser beam source is disposed for each of the photoconductive drums 1A, 2A, 3A and 4A. Each laser beam source is driven to turn on and off in accordance with the calculated image color data to optically write the image of each separated color on the photoconductive drum.
This type of the image forming apparatus has also another problem such as the fact that an image forming speed is not improved on the single color mode and the multiple color mode. Because the sheet is transferred to on the transfer belt by same speed and same timing between the single color mode and multiple color mode.
For the above problem, a solution is described in Japanese Laid Open Patent Application 04-284,468. The solution is to accelerate a rotation speed of the polygon mirror or a clock speed in order to increase speed of the mono color printing in comparison with one of the full color printing.
However, the rotation speed of the polygon mirror or a clock speed is preliminary set at a maximum selling in order to obtain a full use of its potential performance. It is hard to further increase rotational speed and a clock when the single color mode is selected. Therefore, additional improvement is difficult using the same structure.
Referring to FIG. 3, an image forming apparatus in which an image forming speed is increased without increasing a scanning speed at the time of a subtractive multiple color mode is described.
At the time of a single color mode like a monochromatic printing mode K, movable mirrors MMa, MMc, MMd are arranged at an optical path changeover position and a turnable mirror RM 1 is arranged at the optical path changeover position so that four light beams La-Ld emitted from laser diodes 31a-31d and deflected by a polygonal mirror 33 are all led to a photoreceptive drum 21K. Then, a sheet is supplied at the system speed of four times of a default speed. The four light beams La-Ld modulated by the image data of the (4nxe2x88x923)th, (4nxe2x88x922)th, (4nxe2x88x921)th and 4nth scanning lines of the K are emitted in parallel in coincidence with the supply of the recording sheet.
However, the present inventor identified that the above apparatus has the following four problems. First, the apparatus needs mechanical moving parts by which the position of optical devices (e.g. mirror) are adjusted in the optical path upon the switching the multiple color mode and the single color mode. The switching of the photoconductive drum, which the light beam scanned and adjustment of the optical path are accomplished by the above.
Secondly, an already existing image forming apparatus can not employ this system. If the existing image forming apparatus employs this system, significant redesigns are required. Complex and precise mechanical moving parts are then needed in order to carry out the above adjustment.
A third problem is that the apparatus generates a plurality of light beams by the multiple optical scanning system for originally other color image forming. Each incident angle of the light beams to the photoconductive drum is dynamically changed and each beam spots on the photoconductive drums are distorted. Therefore, the apparatus declines in the quality of the image. Furthermore, shading characteristics are of worse quality.
The fourth problem is that when the print sequence carry out in ascending order by single color print, then a full color print, and a single color print, the apparatus has to precisely move mechanical moving parts such as mirror in the optical system each time. This adjustment of the optical device requires a predetermined time interval. The total performance of the prior apparatus decreases. The image data also send other light source changed from original light source each printing.
To solve the above and other problems, according to one aspect of the present invention, an optical system for an image forming apparatus has a first light source for selectably emitting the number of image forming light beams based upon control signals and a second light source for emitting image forming light beams less than one of said first light source.
According to a second aspect of the present invention, the first light source of this optical system selectively emits multiple light beams and a single beam or selectably emits two beams and a single beam.
According to a third aspect of the present invention, an image forming apparatus has an optical system in which a first light source is selectably emitting the number of image forming light beams based upon control signals and a second light source is emitting the number of image forming light beams less than one of the first light source, photoconductive devices for forming an electrostatic latent image by the optical system, developing devices each arranged in connection with each of the photoconductive devices are supplying a developer to each of the photoconductive devices to develop latent images thereon, and a movable sheet transfer belt contacting with photoconductive members, wherein the developed images may be transferred to an image forming medium.
According to a fourth aspect of the present invention, an image forming apparatus, has an optical system in which a first light source is selectably emitting the number of image forming light beams based upon control signals and a second light source is emitting the number of image forming light beams less than one of the first light source, photoconductive devices for forming an electrostatic latent image by the optical system, developing devices each arranged in connection with each of the photoconductive devices are supplying a developer to each of said photoconductive devices to develop latent images thereon, a movable intermediate transfer belt contacting with the photoconductive members, wherein the developed images may be transferred to the transfer belt and a transfer member for contacting with a sheet, wherein the intermediate transferred images may be transferred to an image forming medium.